1. a. The Field of the Invention
The present invention relates to the art of producing and making use of retro-reflecting elements adapted to reflect a ray of light back substantially along its own incident path, such elements being principally adapted to be partially embedded within a layer of a suitable composition, forming a traffic regulating sign or marker on a roadway pavement to improve nighttime visibility of said sign or maker, expecially when the source of light is provided by a vehicle headlamp system.
2. B. The Prior Art
The provision of retro-reflecting means associated with signs or markers applied and secured to or formed on a roadway pavement has been heretofore considered for better nighttime visibility and safer traffic flow. These means most commonly consist of essentially globular bodies or aggregates of a transparent substance, such as glass, partially embedded and firmly secured in a layer of a preformed tape or strip which is then applied or directly formed on a roadway pavement (such as a traffic lane dividing strip or other marker), and itself forms a part of the road area of said pavement. These partially embedded globules have uncoated part-spherical portions protruding above the upper surface of the layer and the light rays impinge on said portions and are at least in part downwardly refracted within the transparent globules. Reflective coatings or substances are adjacent the embedded portions of the globules for providing retro-reflection of the refracted rays.
According to current art, said retro-reflection requires a single or plural focusing of the light beams within the globule. Said requirement leads in turn to the use of glasses of sufficiently high and even very high refractive index, and it is known that such glasses are costly and have an undesirably low resistance to impacts, to wear and to weather.
In the British patent specification No. 1,343,196 (essentially corresponding to French Pat. No. 2,086,257), a mode for improving the retro-reflectivity of such globules has been disclosed and discussed. Reference is herein made to such patent literature for acknowledgement of this art. There has been disclosed an approach to the problem relating to retro-reflection of light rays which impinge on the exposed and uncoated part-spherical portions under an angle of incidence near 90.degree.. This occurrence is realistic when the sign, on a roadway pavement, is impinged by rays emitted from a sufficient distance by the headlamps of a vehicle, and the motorist can detect the sign from the retro-reflected light only. This prior approach involves a rather complicated and critical system of multiple focusing and refractions, which requires making use of transparent bodies and binders correlated by noticeable relative refractive indexes, and a precise positional relationship.
It is further known by optics that when a light ray crosses the interface between transparent substances having different refractive indexes, at an angle of incidence different from 0.degree., this ray is in part refracted and in part reflected. The reflected quantity is a function both of the angle of incidence at the interface and of the difference between the refractive indexes of the two substance (a deeper analysis of the phenomenon is unnecessary). In the prior art aggregates and in the systems considered in view of the object of the invention, both that light which is reflected at the exposed part-spherical portion and that which is reflected within the aggregate, is not available for retro-reflection and is dissipated.
It is therefore an object of this invention to provide a new and advantageous retro-reflecting element which has good ability to reflect a light ray back essentially along its own incident path irrespective of the direction along which such incident ray actually impinges on an exposed part-spherical portion of the same element, and which is not subjected to the above and other limitations.
Another object of the invention is to provide a method for producing such new element.